Covulcanization of conjugated diene-containing butyl with halobutyl and butyl rubber

ABSTRACT

Blends of 5 to 95 wt. % conjugated diene-containing butyl rubber with 95 to 5 wt. % halobutyl or butyl rubber are capable of higher carbon black loading and have faster cure times, unusually high tensile strength and improved green strength. The blends may be cured with dienophilic compounds or sulfur-based cure packages.

This is a continuation, of application Ser. No. 393,349, filed Aug. 31,1973, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the covulcanization of conjugateddiene-containing butyl rubber with halobutyl rubber or butyl rubber.

2. Description of the Prior Art

The expression "butyl rubber" is used in the rubber industry to describecopolymers made from a polymerization reaction mixture having thereinfrom 70 to 99.5% by wt. of an isoolefin which has about 4 to 7 carbonatoms, e.g., isobutylene and about 30 to 0.5% by wt. of a conjugatedmultiolefin having from about 4 to 14 carbon atoms, e.g., isoprene. Theresulting copolymers contain 85 to 99.5% by wt. of combined isoolefinand about 0.5 to 15% of combined multiolefin. The preparation of butylrubber is described in U.S. Pat. No. 2,356,128 which is incorporatedherein by reference.

The polymer backbone of commercial butyl rubber is made up primarily ofisobutylene units, with just a few percent of isoprene units. Theisoprene units contribute the small amount of unsaturation present inbutyl rubber. The basic equation is represented by: ##EQU1## WHICHCOMBINE IN THE PRESENCE OF Friedel-Crafts catalysts to form: ##EQU2##WHERE X + Z REPRESENT THE NUMBER OF ISOOLEFIN UNITS INCORPORATED IN THEBUTYL RUBBER, WHILE Y REPRESENTS THE NUMBER OF INITIAL DIOLEFIN UNITSPRESENT, SUBSTANTIALLY AS RANDOMLY INSERTED UNITS. The conjugateddiolefin, isoprene, loses one olefinic linkage upon its essentiallyrandom incorporation into the polymer backbone.

Thus, butyl rubber, as presently produced, contains only a smallpercentage of unsaturation, in the form of the single double bondassociated with the isoprene residue which is incorporated more or lessrandomly throughout the polymer chain.

It has been discovered that butyl rubber could be produced containingconjugated unsaturation, which is essentially randomly distributed alongthe linear polymer backbone. The general formula may be represented by:##EQU3## WHERE X, Y AND Z HAVE THE VALUES PREVIOUSLY DESCRIBED, THOUGHAT LEAST ONE DOUBLE BOND MAY LAY OUTSIDE THE LINEAR BACKBONE.

This variation may be represented by the formula: ##EQU4##

This new butyl rubber has been termed "high reactivity butyl" (HRB) andencompasses the conjugated diene butyl rubber, regardless of where theunsaturation resides in the chain.

The HRB is more completely described in a copending U.S. applicationSer. No. 228,727, filed Feb. 23, 1972, now U.S. Pat. No. 3,816,371. Oneof the preferred methods of preparing this butyl rubber is described incopending U.S. application Ser. No. 228,728, filed by Francis P.Baldwin, one of the present inventors, on Feb. 23, 1972, which issued asU.S. Pat. No. 3,775,387 on Nov. 27, 1973. Both applications areincorporated herein by reference.

One of the present inventors, Francis P. Baldwin, has described thecovulcanization of blends of from 10 to 90 wt. % conjugated diene butylrubber with from 90 to 10 wt. % high unsaturation rubber, such asnatural rubber, styrene-butadiene rubber (SBR) and the like, incopending U.S. application Ser. No. 228,727, filed Feb. 23, 1972.

Of the many unusual and interesting features of conjugated diene butylrubber, it has recently been discovered that the rubber develops itsmaximum tensile strength at higher carbon black concentrations, whencompared with regular butyl or halogenated butyl rubbers. This isparticularly true with HAF-LS carbon black, where maximum tensilestrength of the conjugated diene butyl occurs with 75-80 parts black per100 parts rubber (phr), as compared with 50-55 phr black with both butyland chlorinated butyl rubber. In addition, the conjugated diene butylrubber cures in about one-fifth (1/5) the time necessary to cure butylor halobutyl rubber, even using relative "mild" cure packages.

SUMMARY OF THE INVENTION

It has now been discovered that the above-described tensile strength andfast cure advantages of conjugated diene butyl rubber can besynergistically achieved in butyl and halobutyl rubber, by blendingtogether from 5 to 80 wt. % conjugated diene-containing butyl rubber,with from 95 to 20 wt. % of butyl or halobutyl rubber. The blend isreinforced with carbon black and cured with sulfur-type cure systems orby use of a dienophilic compound, such as trimethylolpropanetrimethacrylate.

The resulting carbon black loaded, but uncured blends have unusuallyhigh green strengths, and can be formed into inner tubes. A particularlyuseful advantage of the blend is its development of a complete cure in arelatively short period of time, at commercially acceptabletemperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is well known that polymers prepared with different monomers arerarely, if ever, compatible in the physical sense. On the other hand,polymers prepared from predominantly one monomer by a given mode ofenchainment and containing only minor structural perturbations arisingfrom copolymerization or chemical modification can be expected to becompatible. Once the barrier of physical incompatibility is removed, onecan anticipate full utilization of any possible chemical synergismsarising from the blending together of physically compatible polymerscontaining minor chemical modifications.

We have found that the blending together of isobutylene based polymerscontaining conjugated diene groupings with other isobutylene basedpolymers containing simple olefinic linkages and/or allylicallysubstituted halogen atoms can lead to important consequences, both of atechnical and economic nature. Thus, the blending together of minorquantities of conjugated diene-containing butyl rubber with majorquantities of regular butyl rubber can lead to faster than anticipated(on an additive basis) cure rate, higher than anticipated modulus andtensile strength, much improved green strength after hot mixing andbetter than anticipated interaction with carbon black.

The high reactivity butyl rubber, containing the conjugated dieneunsaturation, is prepared by dehydrohalogenation of halogenated butylrubber.

Halogenated butyl rubber has been developed in recent years and hascontributed significantly to the elastomer business. A method ofpreparing halogenated butyl rubber is described in U.S. Pat. No.3,099,644, which is incorporated herein by reference. Both chlorinatedand brominated butyl rubber are typified by: ##EQU5## where x, y and zhave the same values as for butyl rubber, described above, though thisstructure is but one of several which can be formed, depending on theconditions of halogenation, the halogenating agent used, etc.

Halogenated butyl rubber is commercially available and may be preparedby halogenating butyl rubber in a solution containing 1 to 60% by weightbutyl rubber in a substantially inert C₅ -C₈ hydrocarbon solvent such aspentane, hexane, heptane, etc. and contacting this butyl rubber cementwith elemental halogen for a period of about 2-25 minutes. There is thenformed the halogenated butyl rubber and a hydrogen halide, the copolymercontaining up to one or somewhat more, especially in the case ofbromine, halogen atom per double bond initially present in thecopolymer. This invention is not intended to be limited in any way bythe manner in which butyl rubber is halogenated or dehydrohalogenatedand both chlorinated and brominated butyl rubber are suitable for use inthis invention.

Illustrative of halogenated butyl rubber is Exxon Butyl HT 10-68 (achlorinated butyl rubber which before halogenation analyses ˜1.8 mole %unsaturation and a viscosity-average molecular weight of about 450,000).However, for the purposes of this invention, it is preferred that thebutyl rubber starting material have incorporated therein from about 0.5to 6% of combined diolefin, more preferably 0.5 to 3%, e.g., about 2%.

Conventional high molecular weight butyl rubber generally has a numberaverage molecular weight of about 25,000 to about 500,000 preferablyabout 80,000 to about 250,000, especially about 100,000 to about 200,000and a Wijs Iodine No. of about 0.5 to 50, preferably 1 to 15. Morerecent low molecular weight polymers are prepared to have number averagemolecular weights of from 5,000 to 25,000 and unsaturation expressed asmole %, of 2-10.

A particularly advantageous method of preparing conjugateddiene-containing butyl polymers comprises heating a solution ofhalogenated butyl rubber in the presence of a soluble metal carboxylate.Suitable metals are the polyvalent metals of Groups Ib, IIb, IVa, andVIII, of the Periodic Table, having a relatively high first ionizationpotential and whose halides are soluble in the hydrocarbon reactionmedium at the reaction temperature. Typical of these are zinc, iron,mercury, nickel, copper, tin and cadmium carboxylates.

Especially useful are the soluble carboxylic acid salts of zinc (e.g.,zinc salts of naphthenic acids). While useful in preparing thecompositions of the present invention, potential toxicity problems whichcould be encountered in practicing the present invention might limit theuse of certain metals, such as cadmium and mercury salts, for example.

Zinc carboxylate is the most preferred catalyst in the presentinvention. However, in dehydrohalogenating the halogenated butyl rubber,according to the present invention, zinc chloride is thought to be aby-product in the reaction. Zinc chloride, being an effectiveFriedel-Crafts type catalyst, may lead to molecular weight degradationor crosslinking of the halogenated polymers, depending on the structureof the polymer, the solvent employed or the reaction conditions.

This difficulty is overcome, in the present invention by having presentin the reaction zone a metal oxide, hydroxide or carboxylate whosehalogen salt is insoluble in the reaction medium.

It has been found that the mole percent of conjugated diene unsaturationin the dehydrohalogenated butyl, ranges from about 0.5 to about 2.5.

The conjugated diene-containing butyl rubber may be cured by a varietyof methods, such as sulfur, sulfur-containing curing agents,polyfunctional dienophiles such as acrylic and methacrylic acid esters,and the like.

It has been found that two disadvantages of butyl rubber in commercialapplications, i.e., slow cure rate and poor reinforcement capacity, canbe overcome by blending butyl rubber with as little as 5-20 wt. % of theconjugated diene-containing butyl rubber. A synergistic effect permitsthe maintenance of a high proportion of the reinforcement/curingadvantages of conjugated diene butyl in blends containing small amountsof this high reactivity, conjugated diene butyl rubber.

A preferred amount of HRB in the blend, with either butyl or halobutylrubber, ranges from 5 to 80 wt. %, based on total rubber in the blend.Preferably, the amount of HRB ranges from 10 to 60 wt. %, when asulfur-based cure system is used in vulcanizing the carbon black loadedrubber blend. If a polyfunctional dienophile is used to vulcanize theblend, the preferred amount of HRB, in the blend, ranges from about 60to 80 wt. %, or if desired, somewhat higher.

Carbon black fillers are well known in the art. However, a particularlyuseful reinforcing black is HAF-LS Black. Other standard ingredients arealso normally added to the blended rubber compound prior tovulcanization. These ingredients, as well as the carbon black, are usedin essentially commercially accceptable amounts. In some applications,however, advantage can be taken of one of the features of the presentinvention, by use of relatively high loadings of carbon black to achievehigh tensile strength compounds. Loadings of up to 70 to 85 phr ofblack, or higher, are particularly useful for this purpose.

Use of less "potent" accelerators, such as the sulfenamides, areparticularly useful in sulfur-type vulcanization of the present blends.However, for some applications, the thiuram/thiazole type acceleratorsmight be usful. Typical of the sulfenamides is Santocure, which isN-cyclohexyl-2-benzothiazole sulfenamide. The more mixed activeaccelerators may be represented by Altax (benzothiazyl disulfide) andEthyl Tuads (tetraethylthiuram disulfide).

When the polyfunctional dienophiles are used to vulcanize the blend,cures may be obtained at temperatures ranging from 200° up to 420°F.When using the HRB/halobutyl blends, zinc oxide may or may not be used.Zinc oxide tends to cure halobutyl and use of ZnO would depend onwhether the dienophile were needed, by itself, as the vulcanizationagent, or whether it would be used as a cure enhancement agent.

The polyfunctional dienophiles such as the acrylic and methacrylic acidesters are well known crosslinking monomers, used in enhancing peroxidecrosslinking of ethylene-propylene rubber, and in preparing coatingsusing free radical initiators, such as high energy radiation, UV, heat,etc. Typical of these are trimethylolpropane trimethacrylate, 1.6-hexanediol diacrylate, 1.3-butylene glycol dimethacrylate, pentaerythritoltetraacrylate, trimethylolpropane triacrylate, polyethylene glycoldiacrylate, triethylene glycol dimethacrylate, and diethylene glycoldiacrylate. These may be purchased from the Sartomer Company, WestChester, Pennsylvania.

The inventors have only listed a partial sampling of the manypolyfunctional dienophiles, and are not thereby limiting their inventionto those listed.

The invention will be more completely understood by reference to thefollowing examples:

EXAMPLE 1

In order to demonstrate the preparation of the high reactivity,conjugated diene-containing butyl rubber, the following experiment wasconducted.

A one liter glass, vapor jacketed reactor, fitted with stirrer andreflux condenser on reactor and jacket, was charged with 50 grams of achlorinated butyl rubber (Chlorobutyl HT-1068, manufactured by ExxonChemical Company, U.S.A.) in 500 cc of xylene, 4 g. zinc naphthenate,0.5 g naphtenic acid, and 3 g. powdered lime (CaO). The zincnaphthenate, naphthenic acid and CaO were added after the rubber wasdissolved. The reactor was then blanketed with nitrogen.

The vapor jacket, also charged with xylene, was then brought to reflux,leading to a reactor temperature of about 135°C. After 0.5, 1, 2 and 4hours of heating, 75 ml samples were withdrawn from the reactor, placedin centrifuge tubes, diluted with approximately 30 ml of hexane andcentrifuged.

The clear fluid in the tubes was then slowly poured into rapidlyagitated acetone to precipitate the polymer. The precipitate was thenstored for 12 hours under 200 ml acetone containing 0.2 g. of anantioxidant. The polymer was dried in a vacuum oven at about 50°C for 16hours.

Samples were submitted for chlorine analysis, the results of which arein Table I.

                  TABLE I                                                         ______________________________________                                        CHLORINE ANALYSIS                                                                    Reaction Time,                                                         Sample Hours          % Cl      % Cl Removed                                  ______________________________________                                        A      0              1.14      0                                             B      0.5            0.24      78.8                                          C      1.0            0.21      81.5                                          D      2.0            0.14      87.6                                          E      4.0            <0.06     >97                                           ______________________________________                                    

The material remaining in the reactor, which was allowed to cool toambient temperature after 4 hours of heating at 135°C was removed fromthe reactor and diluted with about 600 ml hexane, the solids settled bygravity and the polymer contained in the clear supernatant fluidprecipitated in acetone. The precipitate (designated Sample F) wastreated in the same manner as the withdrawn samples in Table 1.

After drying, the sample F was compounded as follows:

    Polymer Sample F       100 parts                                              m-phenylene-bis-maleimide                                                                            4.5                                                

A sample of this material was placed in a mold in a curing press for 60minutes at 100°C. On removal of the crosslinked vulcanizate, a samplewas immersed in cyclohexane. At equilibrium the sample exhibited aswelling ratio (wt. of sample + wt. of solvent/wt. of sample) of 3.62,indicating a highly crosslinked network.

Drying and reweighing of the swollen sample indicated insolubilizationof greater than 96% of the polymer.

EXAMPLES 2-6

Using a conjugated diene-containing butyl rubber, prepared in the mannerof Example 1, several compounded blends were prepared with a chlorinatedbutyl rubber. The halobutyl used was CHLOROBUTYL HT-1068 (as used inExample 1). The conjugated diene butyl contained 0.21% of chlorine; hada dilute solution viscosity (DSV) ratio of 0.866/.747 (.5/1) and a mole% of conjugated diene unsaturation of 1.45.

The blends were each heated in a curing mold at 320°F for times rangingfrom 20 minutes up to 160 minutes. Upon completion of the exposure tocuring temperatures, the specimens were placed in cyclohexane and swellratio determined, as in Example 1.

The compound ingredients and the resulting swell ratio are shown inTable II.

                                      TABLE II                                    __________________________________________________________________________    DIENOPHILE-CURED RUBBER BLENDS                                                CONJUGATED DIENE-CONTAINING BUTYL (HRB)/CHLOROBUTYL HT 1068                   EXAMPLE      2       3       4       5       6                                __________________________________________________________________________    HRB         100.0   95.0    90.0    85.0    80.0                              HT 1068              5.0    10.0    15.0    20.0                              SRF Black   ------------------------------------50.0----------------------                --------------                                                    Stearic Acid                                                                              ------------------------------------ 2.0----------------------                --------------                                                    SR-350(.sup.a)                                                                            ------------------------------------ 2.5----------------------                --------------                                                    Cure, min. at 320°F.                                                               Swelling Ratio (Wt. % Solubles)(.sup.b)                           20          6.21 (11.2)                                                                           6.10 (12.5)                                                                           5.90 (12.6)                                                                           6.28 (13.4)                                                                           6.12 (14.8)                       40          4.17 (5.4)                                                                            4.09 (5.9)                                                                            4.07 (6.0)                                                                            4.21 (7.5)                                                                            4.19 (6.7)                        80          3.36 (3.4)                                                                            3.30 (3.6)                                                                            3.26 (3.6)                                                                            3.36 (3.9)                                                                            3.38 (4.4)                        160         2.97 (2.6)                                                                            2.90 (2.8)                                                                            2.91 (2.9)                                                                            2.96 (3.1)                                                                            2.98 (3.2)                        __________________________________________________________________________    (.sup.a)Trimethylolpropane trimethacrylate                                            Swollen Wt.                                                           (.sup.b)Cyclohexane                                                                         (% Wt. loss after swelling)                                             Dry Wt.                                                           

EXAMPLES 7-10

A series of experiments were conducted to compare the physicalproperties of several blends of butyl rubber (Exxon Butyl 268) with theconjugated diene butyl used in Examples 2-6. The basic compound was:

                         Parts                                                    Rubber                 100                                                    Stearic Acid           1.0                                                    Carbon Black (GPF)     60.0                                                   FLEXON 840 0:1         20.0                                                   ZnO                    5.0                                                    Sulfur                 2.0                                                    Ethyl Tuads            1.0                                                    Altax                  1.0                                                

The above compound represents an "inner tube" formulation, and wasprepared using a Midget Banbury with the batch size adjusted to yieldapproximately 260 cc of product (upside down mixing technique with oiladded first to the black, 5 minute mixing time.) The compounded blendswere vulcanized for varying periods of time at 307°F. The above testsamples were vulcanized and tested according to standard ASTMtechniques.

The results of these comparisons are found in Table III.

                                      TABLE III                                   __________________________________________________________________________    BUTYL/HRB RUBBER BLENDS                                                       EXAMPLE             7     8     9     10                                      __________________________________________________________________________    Butyl, parts        100    90    80    60                                     Conjugated Diene Butyl (HRB), parts                                                                0     10    20    40                                     Green Strength.sup.(a), seconds                                                1) No heat treatment                                                                               12.4                                                                                13.6                                                                                16.5                                                                                18.9                                   2) Heat treatment    13.1                                                                                13.7                                                                                51.9                                                                                68.7                                  Cured 15 min. at 307°F.                                                Modulus at                                                                              100%      130   160   175   175                                     (lbs/in.sup.2)                                                                          200%      290   430   500   580                                               300%      500   825   950   1125                                              400%      750   1225  1405  1630                                              500%      1075  1615  1805  2010                                    Tensile Strength (lbs/in.sup.2)                                                                   1750  1940  2045  2055                                    % Elongation        685   600   580   515                                     Shore Hardness       50    47    48    47                                     Cured 45 min. at 307°F.                                                Modulus at                                                                              100%      225   265   275   260                                     (lbs/in.sup.2)                                                                          200%      535   730   800   840                                               300%      820   1475  1355  1495                                              400%      1140  1650  1845  --                                      Tensile Strength (lbs/in.sup.2)                                                                   1580  1720  1920  1965                                    % Elongation        525   520   415   390                                     Shore Hardness       55    52    52    50                                     __________________________________________________________________________     .sup.(a) Heat treatment is achieved by holding masterbatch in sample mold     for 45 minutes at 307°F.                                          

GREEN STRENGTH TEST

The green strength test, used in Examples 7-10, was developed by J. A.Rae, Esso Research and Engineering Company, and is conducted on anInstron testing machine. Using the basic compound formulation above, therubber is compounded on the Midget Banbury, using a load factor of 1.6.The mixing cycle comprises adding black, ZnO, , oil and polymer insequence. The ram is then lowered, followed by mixing for 5 minutes.Cooling water is used to control the dump temperature to 270°F (±10°F).

The batch is then added to a cold mill and worked for about 11/2 to 2minutes. The rubber is molded in a Demattia mold to form a notch whichis 0.176 inches in diameter. Molding conditions are 10 minutes at 212°F.The molded sample is water quenched and stored overnight at 75°F. Thespecimens are cut to a 1/4 inch by 4 inch size from a 3 × 6 inch pad.Care is taken to avoid air bubbles when cutting specimens.

The Instron testing machine is used with a "C" strain gauge cell whichis standardized, immediately before using, with calibrated weights. Thechart speed is set at 5 inches per minute and the strain rate at 20inches per minute. The distance (vertical) between the sample jaws isadjusted to exactly 2 inches. The Instron movable jaw is adjusted sothat after 2 inches of travel (100% elongation) it stops automatically.The stress or load is recorded automatically on the moving chart. Thetime (seconds) for maximum imposed stress to decay 70% is taken as themeasure of green strength.

It is readily seen that the inner tube green strength was increased whenthe conjugated diene-containing butyl rubber was blended with theregular butyl rubber. The increases are relatively small unless themasterbatch is heat treated. Although an exhaustive testing was notcarried out on this aspect, the inventors feel that the green strengthof an actual commercial tube compound comparable to Example 9 would besomewhere between 16.5 and 52 lbs/in² with at least a partial heattreatment effect being created by the extrusion and factory mixingoperations.

Using the Table III data for 300% modulus, and determining thestress-strain curve, it was found that the blend (10% of Example 8) hada distinct cure rate advantage over the all-butyl compound of Example 8.It was found that in order to achieve essentially equivalentstress-strain values, it was necessary to cure the 100% butyl 3 timeslonger than the 90% butyl (Example 8).

Moreover the 90% blend (Example 8), in addition to having more "length"and strength than its all-butyl rubber counterpart, is more rubbery, asdetermined by the method of A. M. Gessler, Rubber Age, 94, 602 (1964).Following the procedure outlined there, it was found that thestress-strain curve of 90% blend was "sigmoidal" in shape.

The use of more than 10% conjugated diene-containing butyl in the blendsimply magnifies the differences which are shown in Table III.

What is claimed is:
 1. A compositon of matter comprising a curable blendof from 5 to 95 weight percent (wt. %) conjugated diene-containing butylrubber consisting essentially of a copolymer consisting of from 85 to99.5% by weight of an isoolefin having from 4 to 7 carbon atoms,combined with 15 to 0.5% by weight of a conjugated diolefin having from4 to 14 carbon atoms, containing in the linear backbone conjugated dieneunsaturation, the copolymer having a number average molecular weight offrom about 5,000 to 500,000 and from 95 to 5 wt. % of a rubber selectedfrom the group consisting of butyl or halogenated butyl rubber.
 2. Thecomposition of claim 1, wherein there is also present in the curableblend from 50 to 85 parts, per hundred parts rubber, of a carbon black.3. The composition of claim 2, wherein the carbon black is HAF-LS carbonblack.
 4. The composition of claim 1, wherein the curable blend containsa sulfur-type vulcanization system.
 5. The composition of claim 1,wherein the curable blend contains a polyfunctional dienophilicvulcanization system.
 6. The vulcanized compositon of claim
 1. 7. Thecomposition of claim 5, wherein the polyfunctional dienophile isselected from a di- or higher acrylic or methacrylic acid ester.
 8. Thecomposition of claim 1, wherein there is from 10 to 60 wt. % conjugateddiene-containing butyl rubber.
 9. The composition of claim 5, whereinthere is from 60 to 95 wt. % conjugated diene-containing butyl rubberand from 40 to 5 wt. % halogenated butyl rubber.
 10. The composition ofclaim 9, wherein the halogenated butyl rubber is chlorinated butylrubber.